Thermionic cathode treatment



June 17, 1941. M. BENJAMIN THERMiONIC CATHODE TREATMENT Filed Aug. 12,1939 Patented June 17, 1941 UETED THERMIONIO GATHODE; TREATIVIENT MarkBenjamin, ,Wembley, England, assignor to General Electric Company, acorporation of New York Application August 12, 1939, Serial No. 289,923In Great Britain August 17, 1938 3 Claims.

The present invention is an improvement in thermionic cathodes of theactivated type in which the base material consists of nickeliferousmaterial, that is, nickel or an alloy of nickel.

Oxide-coated cathodes, sometimes known as Wehnelt cathodes, whenemployed in gaseous discharge lamps, in cathode ray tubes, or in otherthermionic devices, after a period of use lose their electronemissivity. In some cases cathodes which no longer are effectivelyelectron-emissive can be observed still to be coated with oxide.

I have discovered that one of the causes of loss of activity ofoxide-coated cathodes is the deleterious effect of sulphur which ispresent ordinarily in the metal base or core. Commercial nickel containsappreciable quantities of sulphur which known metallurgical processesare not capable of removing sufliciently to avoid such deleteriouseffect on cathodes made therefrom.

In accordance with the method step constituting my invention, residualsulphur is removed substantially entirely from articles of nickel, suchas nickel wire strip or sheet material, or in any event to such extentthat any remainder is harmless to electron emission of cathodes madefrom such material.

The accompanying drawing illustrates conventionally a thermionic deviceto which my invention is applicable. In carrying out my invention thenickeliferous material is heated in contact with a reagent which isreactive towards sulphur at high temperature. For example, the wirestrip, wire or other article of nickel, or one of its alloys, is heatedat a temperature of about 1000 C. while embedded in an oxygenousalkaline earth compound which is sulphur-reactive, the heatingpreferably taking place in an inert atmosphere or in a vacuum. Finelydivided carbonate of barium or other alkaline earth metals, andsimilarly the oxides of such metals in powdered form, may be used as thesulphur-reactive reagent.

The length of time of heating will vary with the thickness of thenickeliferous material. For a material having a thickness of about 0.5millimeter or less, good results can be obtained by heat treatment underthe described conditions continued for about 1 hour.

Nickel or nickel alloy metal, after having been subjected to thisprocess, is fabricated into form suitable for the production of cathodesand is coated with activating material, for example, a

carbonate of barium, strontium, or calcium, or an.

appropriate mixture. The coating subsequently is converted to an oxideand is treated to become active or electron-emitting in accordance withwell understood methods.

The drawing illustrates a rectifier as one form of thermionic cathodedevice to which my invention is applicable. It comprises a container Iin which is mounted a cathode assembly 2 and an anode 3. 'The cathodeassembly comprises a heat shield 4, in which are disposed a heater 5 andan oxide-coated ribbon cathode 6 which is wound on a support 1. Thecathode is treated in accordance with the present invention. The heaterand ribbon cathode are connected electrically as usual to externalcontacts 8 which are mounted on a base 9. The anode 3 is connected to anexternal contact Ill. The container contains an appropriate gas filling.A charge of mercury (not shown) which furnishes vapor during operationcommonly is used. Electrons emitted from the heated cathode 6 passthrough an opening in the shield 4 (not shown) to the anode 3.

Cathodes made from sulphur-free metal have longer useful operating livesin thermionic devices than cathodes containing cores or base metal madeof commercial metal which contains sulphur.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The step in the process of manufacturing oxide-coated cathodes whichconsists in subjecting the nickeliferous base metal for such cathodes toa heat treatment at about 1000 C. while packed in oxygenous alkalineearth compound until the residual sulphur in such metal has beensubstantially eliminated.

2. A step in the manufacture of oxide-coated thermionic cathodescomprising nickeliferous base metal which comprises heating base metalfor such cathodes to about 1000 C. while packed in barium carbonate andpreliminary to coating such material with an oxide layer.

3. The method of manufacturing cathodes for electric discharge devicesfrom nickel bases containing sulphur which consists in subjecting saidbases to a purifying treatment for aboutone hour at about 1000" C. whilepacked in oxygenous alkaline earth compound whereby sulphur issubstantially eliminated and thereafter coating said bases withactivating material.

MARK BENJAMIN.

